1. Technical Field
The present invention relates to a method of producing a protective coating for a substrate and, in particular, to a method of producing a protective coating for a substrate made of a light metal and alloys thereof to prevent corrosion of the substrate.
2. Related Art
Light metals, especially aluminum, are being used to an ever increasing extent for technological applications due to their very low specific gravity. However, a disadvantage of light metals is their propensity to corrode, which is a result of their electrochemically base metal nature. As a consequence, they are generally provided with protective layers to prevent or minimize corrosion.
One known method of this type involves depositing metal layers onto the light metal using a currentless electroplating process, or an electroless plating process. This method is of particular interest where heavy demands are imposed on the outer surface, such as in decorative surfaces. Examples of such applications include light metal wheel rims or wheel trims. Due to the heavy demands imposed on such decorative features, especially wheel trims, it is generally desirable to provide a chrome-plated exterior surface. Although known chrome-plated wheel rims of this type do initially meet these decoratively heavy demands, they are very susceptible to damage and thereby rapidly lose their attractiveness.
DE 196 21 881 A1 discloses chrome-plating car wheel rims by applying a primer layer of powder or wet lacquer, drying the layer, applying a layer of a wet synthetic lacquer, drying the synthetic layer, and finally chrome plating the surface. These varied manufacturing steps are extremely laborious and require the product to be repeatedly stored in other devices during the intermediate steps. In addition, it is necessary to allow time for the drying processes.
DE 195 39 645 A1 discloses a light metal rim for a car wheel coated by applying a glossy varnish to the rim, followed by an intermediate coating in the form of a powdered lacquer or bright nickel-plating formed thereover in order to level-out the exterior surface of the structure. A smooth base for a subsequently deposited, electro-platable, decorative bright metallization was thereby created.
For the purposes of galvanizing aluminum, it was proposed in Volume 1, Part 2, Chapter 15, page 1034 of the xe2x80x9cHandbuch der Galvanotechnik,xe2x80x9d published by Heinz W. Dettner and Johannes Elze (1964) that a highly adherent, intermediary metal layer should be applied initially and, as an alternative, it was mentioned that a porous oxide film could also be used. The metallic layers applied after the intermediary were each applied by an electroplating process, which is possible because both the intermediary metallic layer and the porous oxide film are conductive. It had already been established in the same treatise, that the substrate was easily corrodible thereby. Coatings produced in this manner are thus not protective against corrosion, but rather, just the opposite.
Therefore, in concrete and practical terms, the methods from the state of the art of producing decorative layers having good corrosion protective properties, on aluminum for example, appear to be somewhat as follows: a thin zinc layer incorporating a zincate mordant is formed on an aluminum substrate in electroless manner, followed by electroplating directly with copper and applying a duplex or tri-nickel layer using an electroplating process for the purposes of flattening and protecting against corrosion. A thin layer of bright chrome is then formed thereon using an electroplating process.
Such a system of layers provides the aluminum substrate with adequate protection against corrosion only insofar as the layer is not subjected to any form of mechanical damage which extends as far as the metal substrate.
If such a deeply extending form of damage to the layer or the system of layers occurs, then a voltaic cell ensues whereby the outer layer functions as a cathode and the substrate as an anode which will become oxidized.
Although chemically, chromium as such is a very base metal, it will attain a very positive potential due to the formation of a thin oxide layer on its outer surface (referred to as passivation layer). Oxygen will then be reduced on the outer surface which, in comparison to the aluminum exposed by the damage, has a very large surface area. The oxidization process hereby corresponds to a conversion of the metallic aluminum into Al3+. The corrosion of the aluminum at this damaged spot is quite dramatic due to the very large cathode surface area of the chromium oxide. This is referred to as a catastrophic breakdown of the protective layer.
One object of the present invention is to provide an alternative method of producing a corrosion protective coating for a substrate, preferably a light metal substrate, and a corresponding system of layers which is less sensitive to corrosion than known protective coatings.
One embodiment of the present method involves producing a protective coating for preventing corrosion on a substrate of light metal or light metal alloy in which an electrically non-conductive first layer is initially formed on the substrate whereby the non-conductive first layer is produced by anodic oxidization of the substrate, the non-conductive first layer produced by anodic oxidization is then sealed, whereafter a metallized layer is formed on the non-conductive first layer in electroless manner, and a third layer is then formed on the metallic second layer.
Another embodiment is directed to a system of layers formed according to the method. The system of layers includes a substrate of light metal or a light metal alloy, a non-conductive first layer thereon consisting of an oxide of the substrate material, a second layer including one or more metals formed on the first layer by means of a electroless process, and a third layer.
Preferably, a smoothing metallic intermediary layer may be applied between the second layer and the third layer for the purposes of smoothing and leveling. It is particularly preferred that the third layer be a decorative third layer.